"Bosom" is generally used to refer to the chest area of a person, and therefore the singular is usually used. However, there has been uses of the word "bosoms" to refer to the "breasts" of a woman:

2003, Martin Kelner, The Guardian, 7 Apr 2003:
The prevailing look at Aintree was of a well-upholstered woman wearing an outfit about three sizes too small for her; trouser suits so tight you could not only tell if the lady had a coin in her pocket but see if it was heads or tails, and skimpy tops proclaiming proudly that bosoms are back - and this time it's personal.

This seems however, to be a relatively new usage. Wiktionary defines it as:

(in the plural) A woman's breasts. [from 20th c.]

Thus, it's been coined, yes, rather recently.

I suppose, with language changing, it is correct now to refer to "bosoms", although it isn't so, originally.

However, I must make the distinction in this case. "Bosoms" doesn't refer to one person's chest area, but rather the chest areas of several people in general, the way you would say, chins.

Hmm, in this instance, "bosoms are back", one could replace this with any pair reference, as with, "arm/arms", and it would mean the same thing. As in, "Skimpy tops proclaim that arms are back." So I believe this example isn't accurate in this instance.
–
SisterSep 21 '11 at 1:56

There was no insult intended, @Carole-Lynn, just a sci-fi reference joke. I will remove my comment, though.
–
JeffSaholSep 21 '11 at 2:33

@JeffSahol That was what I had in mind as soon as I read this question. The cited passage in this answer isn't quite correct. That is a fashion article, and refers to women, plural, not singular. "Bosoms are back" means that the trend of displaying them more prominently by women's clothing designers has returned.
–
Ellie KesselmanMar 18 '13 at 4:55